We use the simple future tenseto talk about things that will happen at a time later than now.

Using be going to is one way to form the simple future tense, as in, "I am going to watch a movie tonight." Another way to form the simple future tense is with will, as in, "I will see you soon."

In some situations, like when you're making a prediction about the future, you can use either be going to or will. But to talk about something that's already planned or decided, it's best to use be going to, as in, "I'm going to watch a movie later. Want to watch it with me?"